The present invention relates to a zoom lens and, particularly, to an optical system which is suitable as a photographing lens for a television camera. The optical system can have a focal distance range which can be zoomed easily and changed by inserting a lens group for magnification variation into a basic zoom lens.
As a method for shifting a focal distance range of a zoom lens the following approaches have been known:
That is, (1) a so-called converter system to provide an afocal lens system in front of a zoom lens, an angular magnification of which afocal lens system is not 1, (2) a system to mount an attachment lens between a zoom lens and a camera main body, (3) a system to replace a relay part positioned at an imaging plane side in the rear of a zooming part of a zoom lens with another relay part having different focal distance, (4) a double zoom system containing a zoom system within a relay part, and (5) a system (Japanese Patent Application Sho 48-8118) to insert an attachment lens within a relay lens part consisting of two groups.
With respect to these systems, in the systems (1), (2) and (3) it is difficult to instantly change a focal distance, and a back focus of the total lens system can not be retained unchanged. Furthermore, a total lens length will vary. Also the system (4) results in a considerably long total length.
On the other hand, in a system of an attachment provided within a zoom lens system, which is the system (5), even if a total length of a basic zoom lens is a bit longer, the change of a focal distance can be done in an instant, and a back focus can be retained unchanged. Further, a total length of the zoom lens can be retained constant regardless of mounting or dismounting of an attachment lens. Also, the number of lenses used will be smaller than that in the system (4), thus cost will be less.
For such attachment lens system to be provided at an inside of a zoom lens, for example, as shown in "Optical Engineering" Vol. 15, No. 3, p. 231 to 235, two lens groups constituting a relay lens is fixed and a lens group to vary a focal distance is mounted between said two groups, thus changing the focal distance while retaining a back focus unchanged. However, when this kind of zoom lens is used as photographing lens for a color television, the exit pupil of the lens system will be too closely located to an imaging plane position in many cases. This constitutes great disadvantage with respect to shading for color television. That is, in a color television camera, a color separation optical system consisting of a dichroic mirror is placed in front of each image pick up tube. If the position of an exit pupil of a lens system is too close to an imaging plane, the difference in inclination of a light ray contained in the luminous flux impinging into said dichroic mirror becomes large. Therefore, the difference of the properties at a time when a light ray impinges into a dichroic mirror with a large angle of inclination and the properties at a time when the light ray impinges with a small angle of inclination becomes large. Therefore, if the exit pupil is separated away from the imaging plane, the difference in the inclination angle of incidence is lightened and shading is improved.